Chemically amplified (CA) photoresists have achieved almost universal acceptance in the semiconductor industry whenever the highest levels of resist performance are required. The combination of high contrast, high photospeed, and good etch resistance make these materials extremely attractive for high resolution imaging at Deep UV (248 and 193 nm), VUV (157 nm) and EUV wavelengths (13.4 nm).
A typical CA photoresist contains a polymer which undergoes a reaction that results in increased or reduced solubility in a developer. The reaction is catalyzed by acid, which causes a change in an acid-labile moiety which affects solubility. For example, the reaction might make the polymer less lipophilic and/or more polar, or it might diminish cross-linking, or it might deprotect a group which provides solubility, for example an acid group which provides base solubility. As a consequence of the reaction and the resulting change in solubility, the areas exposed to light are removed or left behind in the development step. A typical CA photoresist also contains a radiation-sensitive acid generator system, consisting of one or more components which, upon exposure to radiation in a suitable wavelength range, undergo a reaction which produces an acid. The term “chemical amplification” refers to the fact that one molecule of acid produced by one photon can have an effect on a large number of polymer molecules.
Most CA photoresists require a postexposure bake step where the exposed resist and substrate are held at a temperature above ambient for a period of time. The temperatures achieved in the postexposure bake step serve to allow the reaction between the photogenerated moiety and polymers in the resist to go forward at an acceptable rate. Postexposure bakes tend to have certain disadvantages. For example, they require close control of contaminants and precise timing. For this reason, it has not been common to use CA photoresists outside the context of semiconductor microfabrication, in which close process control and a clean room environment are the norm.
A class of CA photoresists not necessarily requiring post-exposure bake, the ketal resist system (KRS) class, has been disclosed in a number of references detailed below, for example Japan Patent 3,014,350 (Kokai no. Heisei 9-112989). For resists of this class, the acid-labile moieties generally comprise ketal groups.
One application where CA photoresists have not been common is the production of surface holograms for use in the manufacture of diffraction gratings. Diffraction gratings are widely used optical elements which serve, for example, to separate light of different wavelengths with high resolution. Historically such gratings were made by mechanically ruling fine grooves in a suitable material, and even today mechanically ruled gratings continue to be used.
Since the 1960s, diffraction gratings have also been made using photolithography. To make a diffraction grating by photolithography, a layer of photoresist lying atop a substrate is dried and then exposed to a stationary interference fringe field. The fringe field has alternating areas of low and high illumination intensity. The photoresist is then developed so as to remove either the exposed or unexposed regions. The substrate with attached photoresist may then be coated, and may be used directly as a grating or as a master to manufacture additional gratings. For example, the substrate can be etched so that areas that are not protected by photoresist are selectively removed. The resulting etched substrate may be used as a diffraction grating or used as a master to manufacture additional gratings. For more information regarding the use of photolithography to manufacture diffraction gratings, see Christopher Palmer, Diffraction Grating Handbook (5th ed., Thermo RGL 2002), especially pages 43-54.
In making holographic diffraction gratings, certain characteristics common to other photoresist applications are desired. For example, considerable value is attached to a relatively sensitive photoresist requiring comparatively short exposures. Such short exposures are desirable, for example, for reasons of throughput.
There are also some differences between the use of photoresist for semiconductor fabrication and for making holographic diffraction gratings. For example, masks and projection optics are not used to make diffraction gratings because the desired exposure pattern can be generated directly by optical means as an interference pattern (a process referred to as “direct write imaging”). Also, diffraction gratings may be substantially larger than even the largest commonly used semiconductor wafers (300 mm˜1 foot in diameter at the present time).
Furthermore, holographic diffraction gratings are often made with near-ultraviolet light, and it is desired to employ visible light to make such gratings. The use of near-ultraviolet and visible light for holographic diffraction gratings would be in contrast to the general use of ultraviolet light in semiconductor fabrication at the present time.
In light of these differences between semiconductor manufacturing and the manufacture of diffraction gratings, a CA photoresist to be used in the manufacture of diffraction gratings would ideally have the following two characteristics. First, it would have sufficient absorbance in the visible wavelengths commonly used to make such gratings. Second, it would not require post-exposure bake.